1. Field of the Invention
The present invention relates to a power supply device, and more particularly to a power supply device capable of supplying a large amount of power necessary for an entire system operation.
2. Discussion of the Background
In recent years, copiers, printers, facsimile machines, and machines combining these devices employing an electrophotographic process have become multifunctional, resulting in a more complicated structure and a higher maximum power consumption. There has been a tendency to increase the power supplied to a fixing heater in such an image forming apparatus to reduce the waiting time for an operator of the image forming apparatus due to factors in the image forming apparatus, such as the rising time of a fixing unit or a temporary interruption in a printing or copying process in the case of a reduction in the fixing temperature.
At the same time, the power that can be supplied from a normal power line is limited. This poses a large obstacle to the design of such devices. Measures have been taken not to exceed the maximum amount of power that can be supplied from the power line. In a background power supply device and an image forming apparatus, the power supply device estimates the power consumption, and, when the estimated power consumption exceeds the maximum amount of power that can be supplied from a main power source, the power from the main power source and the power from an auxiliary power source are alternately supplied to a certain load using a switching circuit.
In another background image forming apparatus, a constant voltage source circuit is used as an auxiliary power source, and its output voltage is set higher than that of a main power source. A diode for stopping the backflow to the main power source is placed on a power feeding line for feeding power from the main power source to a load. A switch or another diode is placed between the diode and the load on the power feeding line. The output voltage of the auxiliary power source is applied to the power feeding line. As long as the output voltage of the auxiliary power source is higher than that of the main power source, power is supplied only from the auxiliary power source to the load.
According to these known techniques, a power output circuit of a capacitor, that is, a power feeding circuit for feeding power to a load, is structured as a constant voltage source. When the output of an AC-to-DC power source (main power source), which is a constant voltage source, and the output of an auxiliary power source, which is also a constant voltage source, are alternately supplied to the load using a switching circuit, there is a voltage change, at the time of switching from one power source to the other, due to the difference in output voltage of the two constant voltage sources. For example, this causes an unstable operation of a motor supplying power, and the motor may stop or rotate unsteadily. Such unsteady rotation of the motor may cause a defect in an image formed by the image forming apparatus. In the case of a color image forming apparatus, a color shift may occur.
In another background power supply device, a first power source (main power source) is a constant voltage source, and a second power source (auxiliary power source) is a constant current source. The output of the first power source and the output of the second power source are connected in parallel, and the power from the first power source and the power from the second power source are simultaneously supplied to a load. Since there is no switching from the one power source to the other, voltage changes due to the alternate feeding are suppressed.
In these power supply devices, a load current combining the currents supplied from the first and second power sources to a 24V load or the current supplied from the first power source to the 24V load is measured, and the output current of the second power source is controlled according to the measured current and the maximum current data (MCD) specified for the first power source. The second power source is required to have high responsiveness to supply a stable supply voltage to the DC load.
The load power varies depending on whether the load is turned “on” or “off” or the switching of the load value. In other words, the load power varies according to the usage of the load. This variation is, however, a long-period (low-frequency) variation and is static. For example, when one of the above-described power supply devices are used as the power source for an electrophotographic copier, the load power in a standby state waiting for a copy instruction is different from the load power in an operating state in which a printing process is performed by driving a power system in response to the copy instruction.
Even in the operating state, document scanner driving, photosensitive member driving, charger driving, sheet feeding, and developer driving are overlappingly performed one after another, resulting in increases and decreases in the load power. These increases and decreases in the load power are steady load changes involved in sequence control operations and are long-period (low-frequency), static changes. A load current including such a load change is referred to as a static load current in this specification.
In contrast, an inrush current appears in the load current when the load is switched from “off” to “on”. A current induced by the reactance of a circuit appears when the load is switched from “on” to “off”. An inrush current or a current induced by switching the load value appears in the load current. These currents are temporary, short-period (high-frequency) changes and are dynamic. These changes are referred to as dynamic load changes in this specification.
In case of such a dynamic load change, the second power source having high responsiveness responds to the dynamic load change and outputs current generating a high-frequency oscillation, inducing hunting. This may generate an excessive peak current or cause an unstable operation of the second power source.